This invention relates to the inspection of weld seams, and more particularly to the ultrasonic inspection of weld seams in structures.
In structures, it is common for structural plates to be joined by welded seams. The welds can be made not only with ordinary ferritic steel, but also with metal that is high in nickel content. Ordinary ferritic steel weld material solidifies in a relatively uniform structure. Weld metal that is high in nickel content, however, solidifies in a face-centered cubic (austenitic) crystal structure that typically has an elongated (dendritic) grain structure.
It has been previously known that ultrasonic testing (UT) can be used to inspect ordinary ferritic steel welds for defects. Arrays of single-element transducers have been mounted on rolling carts that travel along a ferritic steel weld (commonly pipeline girth welds). The transducers are tied to computerized data collection systems that allow quick inspection of the seams.
Unfortunately, however, the dendritic structure of austenitic welds make them much more difficult to inspect by ultrasonic testing. The elongated grains of the structure tend to refract the ultrasonic signals, creating a high level of noise in the collected data. Consequently, the use of UT inspection of austenitic welds has been limited.
It has been found that dual-element, longitudinal-wave transducers can be used to provide useful inspection of austenitic welds. However, unlike single-element, shear-wave transducers, dual-element transducers focus on a particular distance, and do not provide information about as broad an area as covered by a single-element transducer. Consequently, linear scanning of austenitic welds has been limited to situations where the operator is interested in focusing on only a particular part of a weld, such as the fusion line between the weld metal and the base metal.
Where the operator is interested in inspecting the full volume of the weld, a xe2x80x9crasteringxe2x80x9d process is conventionally used. In such a process, the operator probes for defects throughout the thickness of a weld by manual sliding a transducer towards and then away from the weld. Moving the transducer laterally with respect to the weld is necessary to redirect the sound from the transducer through different sections of the weld. Rastering is a slow and awkward process. It appears that some efforts have been made to mount dual-element transducers on carriages for automated UT inspection. However, determining the transducer arrays that provide reliable results has been difficult.
Consequently, where the full volume of an austenitic weld must be inspected, those skilled in the art have generally chosen to forego the problems of ultrasonic inspection and rely instead upon radiographic inspection. Tanks for the storage of cryogenic liquids such as liquefied natural gas, for example, are field-constructed from plates joined by austenitic butt welds that require 100% inspection for weld defects. Such welds have been conventionally inspected with photographic film exposed by dangerous levels of radiation. Such radiographic inspection is a potential safety hazard and a time-consuming step that sometimes controls the erection schedule of such tanks.
A new method for easily configuring a useful array of dual-element transducers for automated ultrasonic inspection of austenitic weld seams has been developed. The new arrays facilitate ultrasonic inspection of austenitic weld seams, making it an attractive alternative to radiographic inspection.
It has been found that a useful array of transducers can be readily configured by using a schematic of a section of the weld to be screened. Transducer positions and sound paths are laid out on the schematic, and it is then divided into at least two inspection zones. Transducer housing size and sound paths are selected for each inspection zone, and the maximum and minimum sound path distances from the transducer positions to the boundaries of the heat-affected zones are measured. Focal spot distances are selected that provide adequate sensitivity at both the minimum and the maximum sound path distances. Transducer standoff distances and gate settings are established from the schematic. Test plates are then used to determine transducer gain settings and to confirm adequate width-of-field. The transducers are then mounted on a carriage and used to inspect a test weld seam. It is particularly useful to select a creep wave transducer to inspect the upper portion of the weld metal and heat-affected zones, and to select two sets of gain and gate settings for another transducer, one set of settings for obtaining and analyzing direct reflections of sound from that transducer and the other set of settings for obtaining and analyzing indirect reflections of that sound. To do this, the processor is programmed to analyze the signals from that transducer as though they were signals from two separate transducers.
Applying the standards used in the non-destructive test profession, these methods and apparatus have been found to be capable of reliably detecting both machined flaws and flaws produced by intentional welding errors in austenitic welds.